Heretofore, for a pneumatic tire, especially, a heavy duty pneumatic tire, there has been widely used a method, in which “fine ribs,” provided with a step on a tread surface, are disposed on the tread in order to suppress uneven wear of shoulder ribs. In more detail, this method is for reducing shear forces applying to edges of the shoulder ribs in a driving direction, by producing shear forces in an opposite direction to the driving direction of a vehicle. (For example, Japanese Patent Laid-open No. Hei 2-88311 (pp. 2-3, FIG. 2))
Here, FIG. 1(a) is an expanded plan view showing a part of the tread of a heavy duty pneumatic tire having “fine ribs” such as above. Meanwhile, FIG. 1(b) is a sectional view taken along a line A-A′ of the heavy duty pneumatic tire shown in FIG. 1(a).
As shown in FIGS. 1(a) and 1(b), circumferential grooves 31 are disposed on a tread TR1 on both sides of an equator line CL of the tire, and second ribs 12 are disposed on the outside of the respective circumferential grooves 31. In addition, outside the second ribs 12, straight circumferential grooves 32 are also disposed.
In addition, fine ribs 21 with a step having a different level from the surface of the tread TR1 are disposed in the circumferential grooves 32. Shoulder ribs 11 are, further, disposed outside the circumferential grooves 32.
As described above, for a conventional heavy duty pneumatic tire such as that shown in FIGS. 1(a) and 1(b), a portion of the step formed by the fine ribs 21 and the circumferential grooves 32 allows shear forces to be produced in an opposite direction to a driving direction of a vehicle.
Furthermore, as the methods for suppressing uneven wear of shoulder ribs or the like, there has been used a method, in fact, in which is for suppressing rib sear, that edges of the ribs are worn out locally and unevenly by varying shapes of side surfaces of ribs in a sine pattern along a circumferential direction when viewed from above. (Translated National Publication of Patent Application No. 2002-512575 (pp. 14-15, FIGS. 1-2))
However, the above described method has following problems. One of the problems is that under a circumstance where a force in the width direction, i.e. a lateral force, is more dominant than that in the circumferential direction, application of the method employing the fine ribs with a step having a different level from the surface of the tread is hardly effective in suppressing uneven wear of a shoulder ribs, especially taper wear of shoulder ribs in which the shoulder ribs are unevenly worn out in a tapered shape.
Here, FIG. 1(c) is an enlarged view showing a part of a cross-section of the conventional heavy duty pneumatic tire shown in FIG. 1(b) in the width direction of the tread (in the direction of A-A′). Referring to FIG. 1(c) as an example, this problem will be described in details. In a case where a force, i.e. a lateral force F, in the width direction of the tread TR1 from an outside thereof, is applied to a conventional heavy duty pneumatic tire, since an angle formed by a grove bottom surface 32bt and a rib-side surface 11sd is set to an approximately perpendicular angle, when the tread TR1 contacts with the ground, the circumferential groove 32 is shrunk, starting at a contact point V at which the groove bottom surface 32bt and the shoulder rib 11 contact, and the shoulder rib 11 undergoes a large displacement in the width direction, that is, toward an interior of the tread TR1. The conventional method, by which the uneven wear is suppressed with the fine ribs 21 and circumferential grooves 32 are in reality not so effective in suppressing an occurrence and development of a wear core on an edge portion 11eg of the shoulder rib 11 due to the lateral force F applied.
There is also another problem that, since the fine ribs 21 are provided in the circumferential grooves 32 in general as shown in FIG. 1(c), effective groove areas of the circumferential grooves 32 are reduced and drainage performance is impaired. Therefore, when the ground is wet, a driving stability of a vehicle, i.e. wet performance of a vehicle, is deteriorated.
Moreover, the method, as disclosed in the translated National Publication of Patent Application No. 2002-512575, in which shapes of side surfaces of ribs are varied along a circumferential direction, has a following problem. In some cases, the method is effective to a certain extent not only for suppressing the uneven wear caused by a force applied from the circumferential direction to a heavy duty pneumatic tire, but also, as a consequence, for suppressing the uneven wear caused by a force being laterally applied thereto. However, in general, since the shapes of the side surface of the ribs are varied along the circumferential direction, effective groove areas of the circumferential grooves are reduced. For this reason, the drainage performance as described above is impaired and the wet performance is hence deteriorated. Accordingly, when heavy duty pneumatic tires having such features as described above are mounted on a steering shaft, it is clearly shown up that the driving stability is deteriorated, and therefore it has been a serious problem.
Therefore, in consideration of the problems described above, an object of the present invention is to provide a pneumatic tire, especially a heavy duty pneumatic tire, capable of suppressing the uneven wear due to the lateral force in a width direction of the tread and of enhancing a wet performance.